Existing memories may be classified into volatile working memories such as static random access memories (SRAMs) and dynamic random access memories (DRAMs), and nonvolatile storage memories such as NAND flash memories and hard disk drives (HDDs).
The volatile memories operate at a high speed but consume large power in a standby state due to leakage current. In order to solve this problem, various nonvolatile memories have been studied. Magnetic random access memories (MRAMs) including magnetoresistance devices used as storage elements are nonvolatile memories and may operate at a high speed. Therefore, they are expected to replace the existing working memories.
Magnetic tunnel junction (MTJ) elements are widely used as the magnetoresistance devices. An MTJ element has two state, namely low-resistance state and high-resistance state. Depending on the state of the MTJ element, the resistance of the MTJ element becomes a low resistance or a high resistance state. In a reading operation of the memory, the resistance of the MTJ element is compared with that of a reference resistor, which is between the resistance value of the MTJ element in the low-resistance state and the resistance value of the MTJ element in the high-resistance state. The resistance state of the MTJ element is determined in this manner.
The reference resistor has a resistance value that is intermediate between a mean value of the resistance values of the MTJ elements in the low-resistance state and a mean value of the resistance values of the MTJ elements in the high-resistance state. The resistance value of each MTJ element in the low-resistance state needs to be smaller than the resistance value of the reference resistor and the resistance value of each MTJ element in the high-resistance state needs to be greater than the resistance value of the reference resistor. Most of the MTJ elements meet the two conditions, but in a large-capacity memory device, the two conditions may not be met due to variations in resistance value of the MTJ element.